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Why water is considered as pure substance whereas air is considered as impure substance?
Water is considered pure because it only consists of one type of molecule which is ##H_2O## whereas air is a mixture of Nitrogen and Oxygen and many other which is why its impure.
However if the water contains other compounds like Magnesium, Calcium and Chlorine for example, which comes from the tap then it isn't pure because it has "contaminants", which is sometimes wanted to kill bacteria (Chlorine).
In a laboratory pure water is used, which is made by Deionization or by Distillation of the "contaminated" water, which results in only ##H_2O## molecules being present.Even the water we drink after filtration is not so pure.
So hence the term pure in this context means it is only comprised of one element, like saying for example that the Oxygen in air is pure, whereas air is impure because its not just Oxygen.
Should be noted that when we say pure it isn't 100% pure but close to it in the regions of 99% because it is near impossible to make something fully pure.
Hope I helped :)